No swaps are recommended in this matchup. Grafdigger's Cage doesn't interact with Doomsday's plan: it only stops creature cards from entering the battlefield from graveyards and libraries, and stops spells from being cast from graveyards or libraries. Doomsday's stacked pile is drawn into their hand, and Thassa's Oracle is cast from hand, so Cage does nothing against the kill. Flusterstorm is good against Doomsday's combo turn, since Doomsday is a sorcery and Flusterstorm counters it unless they can pay {1}. It also works against their Brainstorm, Ponder, Personal Tutor and Flow State, so it stays in the deck. Our own game plan of putting a huge threat onto the battlefield with Show and Tell or Sneak Attack can out-race their slower Doomsday assembly if they stumble.